Existing energy absorbing bumpers function in the vehicle speed range of 21/2 to 5 miles/hour. They dissipate the vehicle kinetic energy during a collision with a barrier or another vehicle by means of hydraulic shock absorbers. While a 5 mph vehicle speed is small, it represents a sizable vehicle kinetic energy of 2505 ft. lbs, for example--in a 3,000 lb vehicle weight. Such low speed collisions can occur at the end of a braking cycle from a higher speed, or in parking maneuvers, or tight space situations. It is important then to dissipate the remaining vehicle kinetic energy with minimum deceleration to avoid damage to the vehicle frame and body, or to the barrier.
Mathematical analysis indicates that the most efficient bumper is one that provides a constant vehicle decelerating force over its entire working displacement. This then insures that the maximum amount of vehicle kinetic energy is dissipated as heat in the available bumper travel and with the minimum magnitude of decelerating force.
In a hydraulic shock absorber, this optimum characteristic can be obtained by biasing the pressure relief valve in the absorber piston or cylinder by means of a constant force spring that opposes the valve opening. However, since the optimum vehicle retarding force is a constant one independent of vehicle velocity, there is really no necessity to create this force by hydraulic means. Hydraulic shock absorbers are only necessary whenever the energy dissipating force must be modulated to be some function of the oscillation velocity, as in damping of the vehicle oscillations on its support springs.
On the basis of these considerations, for the constant force collision bumper, the present invention relates to an alternative mechanism that does not require any hydraulic means for creating an energy absorbing force opposing a collision displacement. This mechanism produces a substantially constant vehicle retarding force that is independent of vehicle velocity, as required for the optimum characteristic, and which results in a minimum peak deceleration for a given vehicle stopping distance.